The present invention relates generally to structures for shielding against the penetration of radiated electromagnetic energy, and particularly to joint structures and seams at which energy leakage occurs which is widely disproportionate to the net size of the pinholes or voids through which the radiation passes.
Classified information processing is generally performed on digital electronic equipment, such as computers and digitally controlled instrumentation. These devices transfer data between mainframe units and input and output devices such as keyboard terminals and printers. Because of such transfer of data and information, there is a risk of disclosure because of the availability of sophisticated electronic surveillance devices.
The effectiveness of surveillance devices has increased dramatically in recent years. To meet such sophistication, the shielding of electronic instrumentation has also increased dramatically in sophistication. From the shielding of each individual piece of equipment, the technology has moved to the shielding of entire rooms and even entire buildings.
This shielding is accomplished basically by covering the room or building in its entirety, walls, ceiling and floor, with thin metal sheets or foil. Transmission of radiation through the sheets or through the foil is not a problem. The problem arises at the joints between the sheets, or the seams between foil segments.
Currently, the most effective means of sealing panel joints is welding. Each panel is welded to each adjacent panel. A high quality weld must be performed, as pinholes will pass unacceptable amounts of radiation.
Welding is quite labor-intensive, and whereas it is the most effective joint-sealing means, achieving radiation attenuation of over 120 dB at frequencies of more than 10 GHz, it is, nonetheless, the most expensive technique and, at today's prices, causes the cost of the total enclosure to run between $25 and $35 per surface square foot.
Welding can only be done with plate being unsuitable for foil seams. The best seam sealing system for foil seams involves the use of a metal tape which has been plated with solder on one side. This tape is applied against the seams and the solder is fused to the foil with a hot flat iron. However, since thousands of feet of seam exists even in a moderately-sized installation, this method is also very labor-intensive, and the shielding performance is very sensitive to the quality of the workmanship.
The remaining method relies upon the use of conductive RF tapes, of which there are two basic varieties from a variety of manufacturers. The purpose of the tape is to create as high quality, point-to-point conduction between adjacent plates or foil as is possible, to electromagnetically simulate a continuous sheet of foil at the seams.
The first type of tape achieves the conductivity by the use of an adhesive that is loaded with metal particles. This tape is applied in wide widths across the seams in the foil walls. Unfortunately, the performance of the tape is limited to moderate levels of shielding-effectiveness, approximately 60 dB at the frequencies in question.
The second form of shielding tape comprises a flexible foil backing, the front side of which is covered with an adhesive. To penetrate the adhesive and make good conductive contact with the foil panels, the flexible metallic backing is embossed to produce a grid which penetrates the adhesive and makes reasonably good contact with the underlying foil or sheet metal panel when it is applied under substantial pressure by a roller. However, the metal to metal contact achieved by the embossed grid is still somewhat hit and miss. This creates the primary limitation of the tape's system, a lack of complete conductive continuity across the seam, which is especially critical in the microwave frequency spectrum.